Our previous studies have shown that lymphocytes from rats infected with Nipostrongylus brasiliensis (Nb) formed soluble factors which have affinity for IgE (IgE-binding factor). One of the IgE-binding factors was spontaneously released from lymphocytes obtained 14 days after Nb-infection, and selectively potentiated an in vitro IgE response of rat lymphocytes to an unrelated antigen (IgE-potentiating factor). Another IgE binding factor was formed by incubation with IgE of lymphocytes from the 8th day infected rats, and this factor selectively suppressed the IgE response (IgE-specific suppressive factor). In this proposal, 1) experiments will be carried out to show that the formation of the IgE-binding factors is not restricted to parasite infection. Lymphocytes from rats treated with complete Freund's adjuvant and cells precultured with concavalin A will be incubated for 24 hr in the presence or absence of IgE, and culture filtrates will be assessed for the presence of either IgE-potentiating factor or IgE-specific suppressive factor or both. 2) The physcochemical properties of the IgE binding factors will be studied to elucidate the difference between IgE-potentiating factor and IgE specific suppressive factor. 3) Cellular origin of the two IgE-binding factors will be investigated. 4) Possible relationship between IgE-binding factors and Fc epsilon receptors on lymphocytes will be studied. We believe that both IgE-potentiating factor and IgE-specific suppressive factor are involved in IgE-isotype specific regulation. 5) The role of these factors in the in vivo IgE antibody response will be evaluated.